Aside from the general run of causes, predisposing and exciting, we must recognize the contagious element. Jensen has sought to identify the microbe as a small ovoid bacillus united in pairs, or in long chains. This was present not only in the ingesta, but in the lesions of the mucosa, and in the lymph glands. Its cultures ingested in milk, or injected into the rectum sometimes produced the affection. Microscopically it resembled the bacillus fœtidus lactis, but the latter failed to produce the disease. He looks upon it as a sport of the bacillus coli communis. Perroncito found micrococci, usually arranged in pairs and comparable to the cultures of those obtained from the blood in the pneumonia of calves. The injection of the cultures into the thorax of a Guinea-pig caused pleuro-pneumonia with or without dysentery. The rabbit proved immune. At the necropsy the Guinea-pig like the calf showed the diplococci in the blood. Nikolski who studied the affection in lambs seeks to incriminate both micrococci and bacilli. It is premature to specify any particular microbe as the sole cause of the affection. It seems not improbable that bacterial ferments of one or more specific kinds, which in a healthy animal have no injurious effect, may by special combination, or by growth in a mucosa in a given morbid condition, acquire properties which render them not only violently irritating, but may retain such properties so as to render them actively contagious. In this condition they may overcome the resistance of the most healthy stomach and bowels and attack all young animals into which they may secure an entrance. Certain it is that the infection may persist in the same stable for years, will enter a new herd with a newly purchased cow or calf bought out of a previously infected lot, and will follow the watershed and affect in succession the different herds drinking from a stream as it flows downward. The similarity of the germ found by Jensen to the bacillus coli communis, suggests that in this as in a number of other contagious affections a pathogenic sport from this common saprophyte is at least one of the microbian factors in this disease.